In such power assisted steering systems, a pinion is generally mounted rotatably in a steering housing and is held in engagement with a rack which is axially displaceably arranged in the steering housing. A servomotor which is drive-connected to the rack serves to provide power assistance. In this case, a pressure medium source, comprising a pump and a drive, serves, in connection with a control valve device, to control a flow of pressure medium to and from the servomotor as a function of a movement of the pinion.
European Published Patent Application No. 0 624 135 describes a power assisted rack and pinion steering system of this type. Here, the control valve device is configured as two control valves which have seat valves, the axes of which are substantially perpendicular to the axis of the pinion. If, by rotating the steering handwheel, the pinion is also rotated, counter to the resistance of the rack, then its floating bearing is displaced. This displacement is transmitted to the control valves. In this manner, the control valves are controlled by the steering handle (with the interaction of the rack which offers resistance). As seat valves are used, small movements of the pinion are sufficient to actuate and control the control valves. The movement of the control valves leads to pressure building up in the correspondingly provided cylinder space and thus produces assistive power for deflecting the vehicle wheels by the servomotor.
German Published Patent Application No. 195 41 749 describes a power steering system for motor vehicles having a servovalve configured as a rotational slide arrangement. It is provided here that a steering handle, arranged without a positive coupling to the steered vehicle wheels, actuates a steering angle setpoint generator and interacts with an actuating motor. The servovalve in this case has what is referred to as an open center, i.e., all connections communicate with one another when the rotational slide and the control sleeve assume a central position relative to one another. An electric motor is arranged in this case at one end of the rotational slide and is capable, on account of corresponding pulses, of rotating the rotational slide and the control sleeve in the corresponding direction.
The steering system disclosed in German Published Patent Application No. 195 41 749 may be relatively costly, in particular also for interventions in vehicle dynamics. The energy consumption is higher compared with a conventional power steering system. This also results in corresponding heat losses which have to be dissipated in a costly manner, for example, by the use of an oil cooler. Steering must be carried out with the aid of the electric motor if the hydraulics or an essential component fails. In this case, the electric motor and the corresponding parts have to be designed for this type of use and correspondingly to be of sufficiently large dimensions. The requirement for high dynamics means that the motors become very expensive. The electric motor thus requires a correspondingly large amount of energy from the vehicle-mounted electrical system, with the result that the latter has to be designed to supply sufficient power. The parts relevant to safety, such as the vehicle-mounted electrical system, for example, have to be present in duplicate on account of the safety requirements for such power assisted steering systems, so that higher costs are incurred.
These disadvantages also hold, at least partially, in analogous fashion for the other conventional power assisted steering systems and for the power assisted steering system described in European Published Patent Application No. 0 624 135.
Moreover, it may be disadvantageous in the case of conventional power assisted steering systems that both the instantaneously required energy and the mean energy is relatively high, and in particular the electric motor and the associated parts have to be designed for peak loads (which occur, for example, during parking).
It is an aspect of the present invention to provide, e.g., a reliable, simple and cost effective design of, a steering system, which may ensure that the vehicle may be stopped safely even if the hydraulic system fails.